1. ¹Department of Immunology and Cell Biology, University of Münster, Münster, Germany
2. ²Manitoba Institute of Cell Biology, CancerCare Manitoba, Canada
3. ³Department of Hematology, Oncology and Tumor Immunology, Charité, Berlin, Germany
4. ⁴Institute of Molecular Medicine, University of Düsseldorf, Düsseldorf, Germany
5. ⁵Department of Biochemistry and Medical Genetics, University of Manitoba, Winnipeg, Canada

Correspondence: Professor K Schulze-Osthoff, University of Düsseldorf, Institute of Molecular Medicine, Universitätsstr. 1, Building 23.12, Düsseldorf D-40225, Germany. E-mail: kso@uni-duesseldorf.de

⁶These authors contributed equally to this study.

⁷These authors share equal senior authorship.

Received 8 September 2005; Revised 17 October 2005; Accepted 17 October 2005; Published online 14 November 2005.

Abstract

Apoptin, a chicken anemia virus-derived protein, selectively induces apoptosis in transformed but not in normal cells, thus making it a promising candidate as a novel anticancer therapeutic. The mechanism of apoptin-induced apoptosis is largely unknown. Here, we report that contrary to previous assumptions, Bcl-2 and Bcl-x_L inhibit apoptin-induced cell death in several tumor cell lines. In contrast, deficiency of Bax conferred resistance, whereas Bax expression sensitized cells to apoptin-induced death. Cell death induction by apoptin was associated with cytochrome c release from mitochondria as well as with caspase-3 and -7 activation. Benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone, a broad spectrum caspase inhibitor, was highly protective against apoptin-induced cell death. Apoptosis induced by apoptin required Apaf-1, as immortalized Apaf-1-deficient fibroblasts as well as tumor cells devoid of Apaf-1 were strongly protected. Thus, our data indicate that apoptin-induced apoptosis is not only Bcl-2- and caspase dependent, but also engages an Apaf-1 apoptosome-mediated mitochondrial death pathway.